Why "Perfect"? What is it about Unison, 4, 5 & Octave that is "Perfect"? Is there a pattern I should see or is it an arbitrary thing? I understand moving a half step up or down from a Perfect interval is Augmented or Diminished vs. being Major or Minor, but what's the difference? Why is it Perfect 4th Diminished, not Minor 4th? Why 4th Augmented, not Minor 5th, etc?